Intent matters: how flow and forms of information impact collective navigation.

The phenomenon of collective navigation has received considerable interest in recent years. A common line of thinking, backed by theoretical studies, is that collective navigation can improve navigation efficiency through the 'many-wrongs' principle, whereby individual error is reduced by comparing the headings of neighbours. When navigation takes place in a flowing environment, each individual's trajectory is influenced by drift. Consequently, a potential discrepancy emerges between an individual's intended heading and its actual heading. In this study, we develop a theoretical model to explore whether collective navigation benefits are altered according to the form of heading information transmitted between neighbours. Navigation based on each individual's intended heading is found to confer robust advantages across a wide spectrum of flows, via both a marked improvement in migration times and a capacity for a group to overcome flows unnavigable by solitary individuals. Navigation based on individual's actual headings is far less effective, only offering an improvement under highly favourable currents. For many currents, sharing actual heading information can even lead to journey times that exceed those of individual navigators.

Role of synaptic inputs in determining input resistance of developing brain stem motoneurons.

The contribution of synaptic input to input resistance was examined in 208 developing genioglossal motoneurons in 3 postnatal age groups (5-7 day, 13-16 day, and 18-24 day) using sharp electrode recording in a slice preparation of the rat brain stem. High magnesium (Mg(2+); 6 mM) media generated significant increases (21-38%) in both the input resistance (R(n)) and the first time constant (tau(0)) that were reversible. A large percent of the conductance blocked by high Mg(2+) was also sensitive to tetrodotoxin (TTX). Little increase in resistance was attained by adding blockers of specific amino acid (glutamate, glycine, and GABA) transmission over that obtained with the high Mg(2+). Comparing across age groups, there was a significantly larger percent change in R(n) with the addition of high Mg(2+) at postnatal days 13 to 15 (P13-15; 36%) than that found at P5-6 (21%). Spontaneous postsynaptic potentials were sensitive to the combined application of glycine receptor antagonist, strychnine, and the GABA(A) receptor antagonist, bicuculline. Application of either 10 microM strychnine or bicuculline separately produced a reversible increase in both R(n) and tau(0). Addition of 10 microM bicuculline to a strychnine perfusate failed to further increase either R(n) or tau(0). The strychnine/bicuculline-sensitive component of the total synaptic conductance increased with age so that this form of neurotransmission constituted the majority (>60%) of the observed percent decrease in R(n) and tau(0) in the oldest age group. The proportion of change in tau(0) relative to R(n) following strychnine or high magnesium perfusate varied widely from cell to cell and from age to age without pattern. Based on a model from the literature, this pattern indicates a nonselective distribution of the blocked synaptic conductances over the cell body and dendrites. Taken together, the fast inhibitory synapses (glycine, GABA(A)) play a greater role in determining cell excitability in developing brain stem motoneurons as postnatal development progresses. These findings suggest that synaptically mediated conductances effect the membrane behavior of developing motoneurons.

Role of potassium conductances in determining input resistance of developing brain stem motoneurons.

The role of potassium conductances in determining input resistance was studied in 166 genioglossal (GG) motoneurons using sharp electrode recording in brain stem slices of the rats aged 5-7 days, 13-15 days, and 19-24 days postnatal (P). A high magnesium (Mg(2+); 6 mM) perfusate was used to block calcium-mediated synaptic release while intracellular or extracellular cesium (Cs(+)) and/or extracellular tetraethylammonium (TEA) or barium (Ba(2+)) were used to block potassium conductances. In all cases, the addition of TEA to the high Mg(2+) perfusate generated a larger increase in both input resistance (R(n)) and the first membrane time constant (tau(0)) than did high Mg(2+) alone indicating a substantial nonsynaptic contribution to input resistance. With intracellular injection of Cs(+), GG motoneurons with lower resistance (<40 MOmega), on the average, showed a larger percent increase in R(n) than cells with higher resistance (>40 MOmega). There was also a significant increase in the effect of internal Cs(+) on R(n) and tau(0) with age. The largest percent increase (67%) in the tau(0) due to intracellular Cs(+) occurred at P13-15, a developmental stage characterized by a large reduction in specific membrane resistance. Addition of external Cs(+) blocked conductances (further increasing R(n) and tau(0)) beyond those blocked by the TEA perfusate. Substitution of external calcium with 2 mM barium chloride produced a significant increase in both R(n) and tau(0) at all ages studied. The addition of either intracellular Cs(+) or extracellular Ba(2+) created a depolarization shift of the membrane potential. The amount of injected current required to maintain the membrane potential was negatively correlated with the control R(n) of the cell at most ages. Thus low resistance cells had, on the average, more Cs(+)- and Ba(2+)-sensitive channels than their high resistance counterparts. There was also a disproportionately larger percent increase in tau(0) as compared with R(n) for both internal Cs(+) and external Ba(2+). Based on a model by Redman and colleagues, it might be suggested that the majority of these potassium conductances underlying membrane resistance are initially located in the distal dendrites but become more uniformly distributed over the motoneuron surface in the oldest animals.

Pregnancy alters cardiac receptor afferent discharge in rats.

Reflex effects of cardiac receptor (CR) stimulation are attenuated in pregnant rats. We tested whether CR afferent discharge is reduced during pregnancy by measuring single fiber activity in response to increases in right atrial pressure (RAP) in anesthetized pregnant and virgin rats with sinoaortic denervation. Single fiber activity was isolated from fine filaments of the right cervical vagus nerve. Changes in CR discharge, RAP, and arterial pressure were recorded in response to atrial saline injections (25-300 microl). Resting RAP was similar between groups, and spontaneous CR discharge was similar in pregnant rats (1.95+/-0.21 Hz) and in low-frequency (LF) receptors in virgin rats (1.30+/-0.2 Hz). In virgin, but not pregnant rats, a subset (24%) of CR had higher-frequency (HF) spontaneous discharge (9.91+/-1.19 Hz). During stimulation, the level of RAP above which CR firing increased was significantly higher in pregnant rats, but CR activity was clustered into an LF discharge range. Thus gestation appears to reduce the activity of CR afferents, possibly by increasing stimulus threshold or by selective inactivation of a subset of HF discharging receptors.

Cellular correlates of classical conditioning in identified light responsive pedal neurons of Hermissenda crassicornis.

Cellular correlates of classical conditioning were examined in two recently identified light responsive pedal neurons. The correlates of conditioning consisted of significant decreases in the pedal cells' responses to light (conditioned stimulus) recorded from conditioned animals compared to random controls. Pedal cell P7, which exhibits an inhibitory response to light in naive animals, showed significantly less inhibition during a 5 min light step in conditioned animals as compared to random controls. Pedal neuron P9, which exhibits an excitatory response to light in naive animals, showed significantly less excitation during a 10 s light step in conditioned animals as compared to random controls. The changes in the response to light recorded from pedal neurons P7 and P9 in conditioned animals were not accompanied by any significant changes in membrane potential, action potential amplitude or dark-adapted spike frequency.

Characterization of 4 light-responsive putative motor neurons in the pedal ganglia of Hermissenda crassicornis.

As part of the analysis of the circuitry underlying phototaxis, 4 light-responsive pedal neurons were identified and characterized. The 4 newly identified neurons have been designated as pedal neurons P7, P8, P9 and P10. Pedal cell P7 has an inhibitory response to light, lasting several minutes. Pedal cells P8, P9 and P10 exhibit excitatory 'on' responses to light that last for a few seconds after light onset. Lucifer yellow fills showed that each identified pedal cell has only one process which exits the nervous system through one of the pedal nerves. Various procedures were used to investigate the responses to illumination expressed by the 4 identified pedal neurons. The results indicate that: (1) the light responses are not intrinsic, but are due to synaptic input from other light-responsive cell(s), and (2) the sources of the synaptic input to the pedal cells are the photoreceptors of the eye, and not extraocular photoreceptors or light sensitive neurons within the circumesophageal nervous system.